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TECHNICAL LIBRARY

Extracting BSIM3 Model Parameters Using UTMOST

Introduction

As was detailed in a previous issue of The Simulation
Standard, the BSIM3 Version 2.0 model has been incorporated
into UTMOST. Up to now, users could extract BSIM3
model parameters using either user-defined global or user-defined
local optimization strategies. UTMOST now includes
a dedicated in-built BSIM3 parameter extraction algorithm which
is based on the extraction methodology proposed by Berkeley. The
UTMOST BSIM3 extraction routine can perform both the
measurements and the extraction sequence necessary for the extraction
of an accurate scalable BSIM3 model. This article will describe
the BSIM3 measurement and extraction routine. As an example, the
results of using this routine to extract n-channel and p-channel
scalable models to devices with effective channel lengths down to
0.6µm will be shown.

BSIM3 Measurements

The BSIM3 extraction algorithm requires measurements
from (a) a device with a large drawn length and a large drawn width
(a Large device), (b) devices with a large drawn length and different
drawn widths (W-array devices), and (c) devices with a large drawn
width and a range of drawn lengths (L-array devices). The number
of devices in the W-array or L-array is user-definable and UTMOST
will accept a minimum or one device in each array. For a truly accurate
scalable model it is recommended that a minimum of two devices exist
in the L-array.

A total of four sets of I-V measurements are recorded
by the BSIM3 routine for each device. These measurement sets are
detailed below.

Set 1: IDS VGS
measurements where the gate voltage is swept between defined minimum
and maximum values. The source voltage is grounded and the drain
voltage is set to a low value (i.e. 0.1V). The bulk voltage is
stepped, in six steps, between 0V and a defined maximum.

Set 2: IDS-VDS
measurements where the drain voltage is swept between defined
minimum and maximum values. The source voltage is grounded and
the bulk voltage is set to a low value (i.e. 0V). The gate voltage
is stepped, in five steps, between defined minimum and maximum
values. The minimum gate voltage is determined using the threshold
voltage calculated from the data measured in measurement Set 1.

Set 3: IDS-VGS
measurements where the gate voltage is swept between defined minimum
and maximum values. The source voltage is grounded and the drain
voltage is set to a high value (i.e. 5.0V). The bulk voltage is
stepped, in six steps, between 0V and a defined maximum.

Set 4: I DS-VDS
measurements where the drain voltage is swept between defined
minimum and maximum values. The source voltage is grounded and
the bulk voltage is set to a high value (i.e. -3.0V). The gate
voltage is stepped, in five steps, between defined minimum and
maximum values. The minimum gate voltage is determined using the
threshold voltage calculated from the data measured in measurement
Set 1.

Most of the voltage settings associated with the
BSIM3 measurements are user definable. It is important to note that
the UTMOST BSIM3 routine allows the user to decouple
the measurement and extraction tasks. This is recommended because
probe time is minimized and the user can exercise the maximum amount
of flexibility with respect to the extraction of the BSIM3 parameters.
The BSIM3 measurements should be stored in an UTMOST
log file and used as required. In this case, all of the data measured
and used by the BSIM3 routine is also available for use by UTMOST's
general purpose ALL_DC routines. Thus, needless re-measurements
are avoided in situations where the user wants to perform user-defined
regular or local optimization strategies using the BSIM3 data. The
ALL_DC routine is also very useful for model validation. A typical
plot of the BSIM3 measurement data, as viewed by the ALL_DC routine,
is shown in Figure 1.

Figure 1. Typical BSIM3 measurements
for a n-channel device.

Parameter Extraction

Prior to any parameter extraction the user should
specify values for some of the so-called BSIM3 elementary parameters
like TOX, XJ, NPEAK, and NSUB. BSIM3 expert parameters can also
be specified. The UTMOST BSIM3 extraction algorithm
will now be described.

Step 1: Use the Large device linear
region data from data Set 1, and threshold voltages calculated
from this data, to extract VTH0, K1, K2, U0 (optional), UA, UB,
and UC.

Step 2: Use the W-array linear
region data from data Set 1, and threshold voltages calculated
from this data, to extract K3, DW (optional), and W0.

Step 3: Use the L-array linear
region data from data Set 1, and threshold voltages calculated
from this data, to extract NLX, DL (optional), DVT0, DVT1, and
DVT2.

Step 4: Use the Large and L-array
subthreshold region data from data Set 1, and subthreshold slopes
calculated from this data, to extract VOFF, NFACTOR, and CDSC.

Step 5: Use the L-array and W-array
linear region data from data Set 1 to extract RDS0 (optional)
and RDSW.

Step 7: Use the Large and L-array
data from data Set 4 to extract KETA.

Step 8: Use the L-array saturation
region data from data Set 2, and the output resistances extracted
from this data, to extract PCLM, DROUT, PDIBL1, PDIBL2, PSCBE1,
and PSCBE2.

Step 9: Use the L-array subthreshold
region data from data Set 3 to extract ETA0, ETAB, and DSUB.

Step 10: After the extraction
algorithm is completed UTMOST will optimize any
selected BSIM3 parameters to the saturation region output conductance
data for the L-array devices. In some cases this will improve
the quality of the extracted parameters considerably. If the user
does not select any parameters for this parameter refinement stage
then this step will be skipped.

The extraction of the U0, RDS0, DL, and DW parameters
is optional. The user can set them to pre-defined values if required.
By default the BSIM3 BULKMOD parameter will be set to 1 for n-channel
devices and 2 for p-channel devices. However, the user has the flexibility
to override these values and set BULKMOD equal to 1 or 2 for either
n-channel or p-channel devices. An example of a BSIM3 linear region
fit for an n-channel device is shown in Figure 2 while a BSIM3 saturation
region fit for a p-channel device is shown in Figure 3.

Figure 2. Example of a BSIM3
linear region fit for an n-channel device.

Figure 3. Example of a BSIM3
satruation region fit for a p-channel device.

Example

The UTMOST BSIM3 extraction routine
was used to measure data and extract a scalable model parameter
set for a range of devices which included device geometries (W/L's)
of 20/20µm, 3/20µm, 20/0.65 µm, 20/1µm,
and 20/2 µm. Both an n-channel extraction and a p-channel
extraction were performed. The extractions proved to be quite accurate
as can be seen from the measured versus simulated device characteristics
in Figure 4 (n-channel) and Figure 5 (p-channel), so global or local
optimization was not necessary.

These plots were generated with the ALL_DC routine
which reads data recorded by the BSIM3 extraction routine. The predictions
of device current and conductance in all regions of normal device
operation are in very good agreement with the actual measurements.

It was decided to hold the contact resistance parameter
at a set predefined value suitable for the devices under test. The
BULKMOD = 1 model was chosen for the n-channel devices while the
BULKMOD = 2 model was used for the p-channel devices. In these examples
no use was made of the UTMOST BSIM3 extraction facility whereby
model parameters can be refined or tuned using optimization after
the regular BSIM3 extraction. This facility would have improved
the model accuracy even further if it had been used. In addition,
user-defined local optimization routines or any of the regular UTMOST
optimization features could also have been used to refine the BSIM3
parameters using the data measured by the BSIM3 extraction routine
[2,3].

Conclusions

UTMOST now has a very powerful in-built
BSIM3 measurement and extraction routine. Scalable BSIM3 models
can easily be extracted with this routine. Extraction examples for
n-channel and p-channel devices from a 0.6 µm CMOS process
were used to demonstrate the effectiveness of the extraction algorithm.
The BSIM3 extraction routine includes an optional parameter optimization
facility which can be used to refine or tune the extracted model
parameter set. The data measured by the new BSIM3 routine can also
be accessed by the general purpose ALL_DC routine. This means that
flexible plotting and optimization features are available for the
measured BSIM3 data subsequent to the BSIM3 extraction.